Aug. 9, 2011 -- New research offers the first direct biologic evidence to prove that genes play a big part in intelligence.

Previous family and twin studies strongly suggest that many characteristics of intelligence are inherited, but no specific genes or gene variants have been linked with intelligence traits.

In the new study, investigators found that many different gene variants with small individual effects are involved in human intelligence.

Researcher Ian Deary, PhD, of the Center for Cognitive Aging at the University of Edinburgh in the U.K., tells WebMD that the study is part of a larger investigation into the causes of age-related changes in memory and intelligence.

“In short, our research teams are trying to find out why some people’s thinking skills age better than others,” he says.

40% to 50% of Intelligence the Result of Genes

In an effort to better understand the contribution that genes make to thinking, reasoning, learning, and other aspects of intelligence, Deary and colleagues from other parts of Europe and Australia tested the DNA of 3,500 people for genetic variations.

More than half a million genetic markers were examined for each study participant using a technique developed by study co-author Peter Visscher, PhD, of Brisbane, Australia’s Queensland Institute of Medical Research.

Following the analysis, the identified genetic variations were correlated with the participants’ performance on tests designed to assess two standard measures of intelligence -- knowledge and problem solving.

The findings suggest that between 40% and 50% of the differences in scores among the study participants resulted from specific genetic differences.

Deary says this is in line with previous intelligence studies involving twins, including those raised in separate families.

He adds that environmental influences and other factors may explain the rest of intelligence.

The study appears online today in the journal Molecular Psychiatry.

“The findings leave a lot of room for environmental influences and for interactions between people’s genes and their environment,” he notes. “It is a start to understanding the relationship between people’s thinking skills and outcomes in life and to understanding why some people cognitively age better than others.”

Many Different Genes Involved in Intelligence

Genome-wide association studies, which scan complete sets of DNA for variations in genes, would not have been possible just a few years ago.

Rapid advances in technology have improved the efficiency and dramatically lowered the cost of genome-wide scans like the one conducted by Deary and colleagues.

“We now have the tools to look at large numbers of genes in large numbers of people at the same time,” says Julio Licinio, MD, of Australia’s National University Canberra.

He says the finding that many different gene variants contribute to intelligence is significant.

“Based on these data, it seems unlikely that we will identify one or two genes that make a very big difference in intelligence,” he tells WebMD.

This makes it less likely that researchers will be able to screen for intelligence or target specific genes in the future to boost intelligence, he says.